Action Potential Studies of Afferent Fibres in the IXth and Xth Cranial Nerves of the Frog

SUMMARY1. Response characteristics of baroreceptors with receptive fields in the pulmocutaneous artery (p.c.a.) were determined in pithed toads by applying pressure steps, ramps, sine waves, and volume infusions into the vascularly isolated and perfused p.c.a.2. The baroreceptors exhibited phasic and tonic discharge thresholds (303 + 2-3 and 36-2 + 28 mmHg respectively) which were above mean arterial pressure values reported for conscious undisturbed toads. They are comparable, in this respect, to non-myelinated mammalian arterial baroreceptors.3. The maximum-phasic and minimum-adapted discharge frequencies of the p.c.a. baroreceptors were low (30 and 2-3 spikes s-1, respectively), but resembled those reported for mammalian non-myelinated baroreceptors when an adjustment was made for the difference in the temperature of mammalian and amphibian preparations.4. The sensitivity of the baroreceptor discharge frequency to pressure (AF/AP) was estimated from pressure-step and pressure-ramp stimuli. Both estimates were greater than those reported for mammalian systemic arterial baroreceptors after the values were normalized to the maximum discharge frequency of the receptors. AF/AP and saturation discharge frequency values estimated from ramp stimuli increased with the dP/dt of pressure ramps.5. The diameter of the p.c.a. was measured by sonomicrometry in toads anaesthetized with urethane. The diameter pulsation was 7-7 + 0 3 % of the mean diameter (3-5 + 0.1 mm) at a mean pulse pressure of 22 + 1 mmHg, and Peterson's pressurestrain modulus was calculated to be 4-0 x 105+0-3 x 105 dyn cm-2, which suggests that the p.c.a. is highly compliant, and in this respect is comparable to the pulmonary artery, but not to systemic arteries, in mammals. Baroreceptor discharge began near the point of peak dynamic compliance (d V/dP) and continued as d V/dP decreased. Increasing the rate of infusion reduced the peak dV/dP, but increased the baroreceptor discharge frequency. 6. The response to sinusoidal oscillating pressure stimuli was distorted by rectification. Increasing the frequency of sinusoidal stimulation over the range 0 05-1 0 cycles s-I reduced the number of spikes per cycle, but increased the mean discharge frequency.

Section: Introductionsupporting

confidence: 71%

Response characteristics of pulmocutaneous arterial baroreceptors in the toad, Bufo marinus.

Vliet

West

1987

“…While the effects of lung inflation on the pressures in the pulmo-cutaneous arches are most likely brought about in a purely mechanical fashion, the possibility of nervous control does exist, in that, in Rana temporaria at least, afferent fibres of the vagus originate in the tissue of the lung and show increasing impulse discharge as the lungs are inflated (Neil, Strom & Zotterman, 1950).…”

Section: Discussionmentioning

confidence: 99%

The blood pressure and the pressure pulses in the arterial arches of the frog (Rana temporaria) and the toad (Bufo bufo)

Simons¹

1957

The account of the distribution of the blood in the Amphibia given in the text-books of zoology, and now generally called the 'classical theory', postulates that the pressures obtaining in the pulmo-cutaneous arches are lower than those in the systemic arches, while the pressures in the carotid arches are higher. Because of these pressure differences it is further postulated that the blood issuing from the ventricle flows first into the pulmo-cutaneous arches and then successively into the systemic and carotid arches.Attempts have been made to check these postulates by measuring the blood pressures and registering the pulse waves in the arterial arches, but the results obtained have in general been unconvincing. This has undoubtedly been due to technical difficulties and the comparative crudeness of the instruments used which, with one exception, have been small manometers of low natural frequency.The development recently of electronic condenser manometers, however, has provided an extremely sensitive, efficient and easily used instrument. It was considered, therefore, that useful information might be obtained by the use of these manometers for measuring vascular pressures in the frog and toad and thereby throwing some light on this problem. METHODSThe animals used were mature specimens of Rana temporaria and Bufo bufo. They were anaesthetized by ether. The depth of anaesthesia aimed at was light in order that the blood pressure should not be depressed unduly. The animal was removed from the ether jar as soon as it was observed that it no longer responded to being turned on its back. On removal, it was pinned to a small cork board, the skin was reflected and cut away ventral to the pectoral girdle which was bisected, thus displaying the heart and arterial arches. The pericardium was slit and removed from around the arterial arches, and the hypodermic needles attached to gauge heads of the pressure-recording machine were introduced, under a binocular microscope, into the selected vessels.

“…Most characteristics of anuran baroreceptors are unknown, since recordings of their afferent activity have largely served to demonstrate the presence and position of baroreceptor endings in the arterial circulation (Neil, Strom & Zotterman, 1950; Ishii, Honda & Ishii, 1966;Ishii & Ishii, 1978;Ishii, Ishii & Kusakabe, 1985). From the study of Ishii & Ishii (1978), however, it is apparent that p.c.a.…”

Section: Introductionmentioning

confidence: 99%

Responses to circulatory pressures, and conduction velocity, of pulmocutaneous baroreceptors in Bufo marinus.

Vliet

West

1987

SUMMARY1. Baroreceptor activity was recorded within the recurrent laryngeal branch of the toad vagus in forty-four preparations. The receptive fields of the receptors were located in the pulmocutaneous artery (p.c.a.), generally within 5 mm of its separation from the truncus. However, the most easily recorded afferents in this nerve were mechanoreceptors which responded to punctate stimulation of the lip of the glottis.2. The conduction velocities of p.c.a. baroreceptor and mechanoreceptive glottal afferents recorded in the recurrent laryngeal nerve ranged from 0 3-07 (0-5+041) m s-1 and 2-2-14-0 (6 8+0 8) m s-1 respectively, which suggests that baroreceptor afferent fibres are non-myelinated and that glottal afferent fibres are myelinated.3. The p.c.a. baroreceptor discharge was largely confined to a period of systole in which systemic and p.c.a. arterial pressure profiles were identical. The maximum discharge frequency, number of spikes per cycle, and the duration of discharge increased, and the discharge latency decreased, as p.c.a. pressures were elevated. The latency of the discharge was pronounced at low p.c.a. pressures, and could be partially accounted for by the conduction time of the baroreceptor afferents to the electrodes (up to 100 ms).4. Carotid and aortic arterial baroreceptor and pharyngeal afferents were recorded in two pharyngeal branches of the vagus nerve which were closely associated with the carotid and aortic arterial arches.5. It is suggested that baroreceptor populations with their receptive fields in the third-(carotid) and sixth-(p.c.a. or pulmonary) arch arteries should be considered homologous in anurans and mammals, but that those of the fourth-(aorta) arch artery should not, since the vagal branches in which their afferents are carried do not appear to be equivalent in anurans and mammals.